refactor: use low priority workqueue for underglow and battery reporting

Blocking operations on the high priority system workqueue may result in
deadlocks, particularly when Bluetooth is in use.

app/src/workqueue.c

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2023 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/device.h>
+
+#include <zmk/workqueue.h>
+
+K_THREAD_STACK_DEFINE(lowprio_q_stack, CONFIG_ZMK_LOW_PRIORITY_THREAD_STACK_SIZE);
+
+static struct k_work_q lowprio_work_q;
+
+struct k_work_q *zmk_workqueue_lowprio_work_q() {
+    return &lowprio_work_q;
+}
+
+static int workqueue_init() {
+    static const struct k_work_queue_config queue_config = {.name = "Low Priority Work Queue"};
+    k_work_queue_start(&lowprio_work_q, lowprio_q_stack, K_THREAD_STACK_SIZEOF(lowprio_q_stack),
+                       CONFIG_ZMK_LOW_PRIORITY_THREAD_PRIORITY, &queue_config);
+    return 0;
+}
+
+SYS_INIT(workqueue_init, POST_KERNEL, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT);